A Multiple Input Multiple Output (MIMO) system refers to a system for improving data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas. MIMO technology includes a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme is suitable for data transmission of a user equipment (UE) which moves at a high speed, because transmission reliability is increased or a cell radius is increased through diversity gain. The spatial multiplexing scheme can increase data transfer rate without increasing system bandwidth by simultaneously transmitting different data.
In a MIMO system, each transmission antenna has an independent data channel. The transmission antenna may be a virtual antenna or a physical antenna. A receiver estimates a channel with respect to each transmission antenna and receives data transmitted from each transmission antenna. Channel estimation refers to a process of compensating for signal distortion due to fading so as to restore the received signal. Fading refers to a phenomenon in which the intensity of a signal is rapidly changed due to multi-path and time delay in a wireless communication system environment. For channel estimation, a reference signal known to both a transmitter and a receiver is necessary. The reference signal may be abbreviated to RS or referred to as a pilot signal according to the standard implemented.
A downlink reference signal is a pilot signal for coherent demodulation, such as a Physical Downlink Shared Channel (PDSCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH), and a Physical Downlink Control Channel (PDCCH). The downlink reference signal includes a Common Reference Signal (CRS) shared among all UEs in a cell and a Dedicated Reference Signal (DRS) for a specific UE. The CRS may be referred to as a cell-specific reference signal. The DRS may be referred to as a UE-specific reference signal.
In a system having an antenna configuration (e.g., a system according to the LTE-A standard supporting eight transmission antennas) developed as an extension of a legacy communication system (e.g., a system based on LTE release 8 or 9), it is necessary to transmit a reference signal for acquiring channel state information (CSI) at a reception side, that is, a CSI-RS.